The disk_hints vary between each IaaS. The disk_hints describe the physical attach point of the disk. The Agent is updated with a mapping of volume ID to attach point.
For example, the AWS implementation of the CPI simply returns a string representing the device block id:

For the Agent to eventually format, partition and mount the newly attached disk, it needs to identify the disk attachment from inside the OS. The Agent can currently identify attached disks based on either their device path, disk's ID, or SCSI volume ID. For example, the sample settings below show that the CPI attached a disk vol-7447851 at /dev/sdd: